Block-level storage in a disk storage array is organized as volumes of logical units (LU). Servers access these disk storage array volumes as blocks. The major metrics for these volumes are:
CAPACITY—amount of available storage (in bytes);
IOPs—Input/Output operations per second (that the volume can handle);
LATENCY—time delay (from request) until data starts; and
THROUGHPUT—data rate for a particular volume.
For reference, a typical disk storage array volume using serial Small Computer System Interface (SCSI) disks may have parameters as follows:
Capacity—100 GigaBytes
Input/Outputs (IOPs)—800 random operations/sec
Latency—2 milliseconds (ms) to 4 ms
Throughput—100 Megabytes/sec
Access to a disk storage array is relatively slow compared to Dynamic Random Access Memory (DRAM) or Solid State Flash (Flash) memory. As mentioned above, a memory access to disk can take several milliseconds while RAM accesses are on the order of nano-seconds and Flash memory accesses are on the order of microseconds.